<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>媒体查询</title>
    <style>
         *{
            padding:0px;
            margin:0px;
            box-sizing: border-box;
        }
        .header,.left,.center,.right{
            border:1px solid black;
            margin-top:5px;
            text-align: center;
        }
        .left{
            float: left;
        }
        .right{
            float: right;
        }
        .center{
            float: center;
        }

    </style>
</head>
<body>
    <header class="header"> header</header>
    <aside class="left">left</aside>
    <section class="center">right</section>
    <section class="right">center</section>
</body>
</html>